Principles of Medicine for Engineers
Part 2: Physiology and its Pathological Perturbations
How your body works, and how it can break
Let’s continue our effort to distill medicine into fundamental (engineering) principles — in Part 1 we set the foundations with what a system was and different levels of understanding the system.
In this post I’m going to talk about two of the core concepts in medicine:
- physiology = how the parts of a healthy body work together when you’re feeling well
- patho±physiology = how the parts of a body work when you’re not feeling well
Friendly reminder: this is mostly for myself for now — trying to put down to paper things that are fresh in mind.
A quick preliminary
To talk about the basic principles of patho±physiology, we have to have one quick preliminary about dimensions.
Dimensions seem magic — but they’re just another word for variable. And each moving part has its own variable.
If we’ve got two variables, we can plot it over two dimensions. Three variables, three dimensions.
But what happens when we have four or more variables? It becomes tough to plot it. But we need to find a way to visualize it because even simple organs have more than four moving parts.
One way we can visualize more than 3-dimensions is to re-plot each dimensions along a single axis:
With this way, we see that we can plot 5+ dimensions pretty easily! (Even infinity dimensions…)
This will become important, later here and in future posts.
Physiology is a beautiful, but flawed, dance
Physiology is the coordination that all the parts of our body have with each other when there isn’t any clear symptoms.
This dance looks different in each person, but what it always has in common: we feel good when it happens.
So if we looked at how all the interactions in the body behaved when this group of people were feeling good, we’d notice some conserved patterns.
In this plot, think of each x-axis tick as an “interaction”. That interaction has some weight in each person, some range of weights that it can take on as that person goes about their day.
Beyond that, there’s a set of ranges of weights that a whole group of people can have even as they’re feeling and functioning completely normally.
Of course, changes in certain parts of physiology don’t always lead to feeling fine.
When a step goes wrong…
Every dimension of physiology can be perturbed — sometimes that perturbation is small and the rest of the body can absorb its effect to keep you functioning the same, but sometimes it’s large enough that your body can’t deal with it, and you start feeling off.
That’s what we call patho±physiology.
An important principle: Patho±physiology tends to be sparse in that very few things are actually deviating from physiology.
Non-sparse deviations tend to be very lethal (think severe trauma or multi-organ failure) or incompatible with life (think genetic disorders that you never see in children because they completely halt fetal development.
Anchor in the output
Maybe the most important point here is that pathology is defined by the output space.
That means: just because someone has a perturbation to their physiology does not automatically mean there’s a downstream pathology or disorder.
Principle: If a perturbation hurts functioning or causes suffering, then it’s a patho±physiology that should be treated.
There are plenty of variations that don’t lead to problematic outputs — these are not considered pathological perturbations to physiology. Even if it leads to friction with societal pressures. In those cases the only sustainable intervention may be societal change, not focused intervention in individuals. That’s a bigger discussion for another
Summary
All the parts of the body work together to make you feel good — this is physiology. When one part is out of sync it can potentially cause problems (but not always) — this is patho±physiology.
Pathophysiology tends to be sparse in the parts of physiology that are perturbed and, as a result, likely to be manageable with targeted interventions. Not all perturbations to physiology are considered patho±physiology — some are just healthy variation = physiology.
These basic principles apply to every system in the body, every organ in the body, and indeed every pathology we try to infer and address in medicine.